1. Field of the Invention
The present invention relates to an image forming apparatus. More specifically, the present invention relates to an image forming apparatus that has a print head unit for printing.
2. Background Information
Image forming apparatuses that have a print head unit for printing have been known conventionally. For example, Japanese Patent Application Publications Nos. 11-10925 and 10-58804 disclose such image forming apparatuses.
Japanese Patent Application Publication No. 11-10925 discloses a printer configured such that a ribbon cassette (ink sheet case) is fixedly positioned within the printer, and a head unit is pressed against a platen (a platen roller) by having a lever (locking member) that is attached to the head unit (print head unit) engage a locking pin provided at the side plate of the printer (an example of an image forming apparatus).
Also, Japanese Patent Application Publication No. 10-58804 discloses a heat transfer recording apparatus (image forming apparatus) in which a locking device (locking member) attached to a housing (frame) engages an ink cartridge (ink sheet case), such that the ink cartridge is fixedly positioned within the heat transfer recording apparatus.
A heat transfer printer is a known example of an image forming apparatus. FIG. 13 is a perspective view showing the entire configuration of a conventional example of a heat transfer printer. FIG. 14 is a front view showing a locking member engaged with an ink sheet case in the conventional example of a heat transfer printer shown in FIG. 13. The structure of an example of a conventional heat transfer printer will now be described with reference to FIGS. 13 and 14.
As shown in FIG. 13, a conventional heat transfer printer has a metal frame 101 having a first side surface 101a and a second side surface 101b, a thermal head unit 102 for printing, a platen roller 103, a platen roller bearing 104 for rotatably supporting the platen roller 103, an ink sheet case 105 for accommodating ink sheets, a locking member 106 for fixedly positioning the ink sheet case 105 in place, a metal shaft 107, two press members 108, a drive gear 109, a motor bracket 110, and an intermediate gear 111. The drive gear 109 has a small gear 109a and a large gear 109b made of a resin and pivots the press member 108 on the first side surface 101a of the frame 101. Furthermore, a motor bracket 110 is attached to the first side surface 101a. 
The second side surface 101b opposite the first side surface 101a of the frame 101 is provided with an ink sheet insertion part 101c through which the ink sheet case 105 is mounted. A concavity 105a that engages the engaging unit 106a of the locking member 106 is provided at a predetermined location of this ink sheet case 105.
The locking member 106 has a supporting part 106b, a plate spring 106c, and a gripping part 106d, as shown in FIGS. 13 and 14. This supporting part 106b is coupled to the second side surface 101b of the frame 101 to pivotably support the locking member 106. Also, the plate spring 106c comes into contact with a bent section 101d of the second side surface 101b of the frame 101, and urges the engaging unit 106a of the locking member 106 downward. The gripping part 106d is configured such that when the gripping part 106d is brought upward when the urging force of the plate spring 106c, the engaging unit 106a is separated from the concavity 105a of the ink sheet case 105. Also, the bent section 101d, which comes into contact with the plate spring 106c of the locking member 106, is formed at the top of the second side surface 101b of the frame 101. The first side surface 101a and second side surface 101b of the frame 101 are provided with insertion holes 101e through which both ends of the shaft 107 are rotatably inserted, as shown in FIG. 13.
The thermal head unit 102 is mounted in between the first side surface 101a and the second side surface 101b of the frame 101, so as to be capable of pivoting around the supporting shaft. A head bottom part 102a provided to the bottom of the thermal head unit 102 is disposed so as to face the platen roller 103. Bent parts 102b pressed on by the press members 108 are formed above both ends of the head bottom part 102a of the thermal head unit 102. The thermal head unit 102 is normally urged in a direction away from the platen roller 103 by an urging member (not shown in Figures).
The two press members 108 are unrotatably coupled to near the two ends of the shaft 107. Shaft supports 107a are formed on both ends of the shaft 107, outside the portions to which the press members 108 are coupled. These shaft supports 107a are rotatably supported in the insertion holes 101e of the frame 101. Also, press springs 108a that apply pressure to the two bend-worked parts 102b of the thermal head unit 102 are affixed to the two press members 108. Furthermore, the press member 108 next to the first side surface 101a of the frame 101 is disposed so as to engage the small gear 109a of the drive gear 109. The drive gear 109 is coupled to the first side surface 101a of the frame 101, and transmits driving force from the intermediate gear 111 to the press members 108. The driving force of a motor (not shown) is transmitted to the large gear 109b of the drive gear 109 via the intermediate gear 111.
In the pressing operation for the platen roller 103 of the thermal head unit 102 in the conventional heat transfer printer described above, the driving force of the motor (not shown) is transmitted to the press member 108 that is on the first side surface 101a side of the frame 101 via the intermediate gear 111 and the large gear 109b and small gear 109a of the drive gear 109. Accordingly, the press member 108 on the first side surface 101a side of the frame 101 thereby pivots while being supported by the shaft 107. As a result, the bent part 102b next to the first side surface 101a of the frame 101 is pressed on by the press spring 108a of the press member 108. Since the two press members 108 are unrotatably coupled to near ends of the shaft 107, the shaft 107 and the press member 108 on the second side surface 101b side of the frame 101 pivot as a result of the pivoting of the press member 108 on the first side surface 101a side. The bent part 102b on the second side surface 101b side of the frame 101 is thereby pressed on by the press spring 108a of the press member 108 next to the second side surface 101b. As a result, the head bottom part 102a of the thermal head unit 102 is pressed on by the platen roller 103 against the urging force of the urging member (not shown).
In the conventional heat transfer printer shown in FIGS. 13 and 14, the locking member 106 is mounted on the second side surface 101b of the frame 101 in order to prevent the ink sheet case 105 from moving in the width direction of the thermal head unit 102 (the direction of the arrow E in FIG. 13). However, with this locking member 106, the engaging unit 106a separates from the concavity 105a of the ink sheet case 105 when the gripping part 106d being brought up, and the ink sheet case 105 therefore moves in the width direction of the thermal head unit 102 (the direction of the arrow E in FIG. 13) if the gripping part 106d is mistakenly brought up during printing. In this case, printing defects tend to result.
Also, in the structure disclosed in Japanese Patent Application Publication No. 11-10925, a locking member for securing the ink sheet case in place in the image forming apparatus is coupled to the print head unit. Therefore, the print head unit separates from the platen roller if the locking member is mistakenly released from a locking pin. In that case, printing defects result.
Also, in the structure disclosed in Japanese Patent Application Publication No. 10-58804, a locking member for securing the ink sheet case in place in the image forming apparatus engages the ink sheet case. Therefore, the ink sheet case moves relative to the image forming apparatus if the locking member is mistakenly released from the ink sheet case. In that case also, printing defects result.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming apparatus that overcomes the problems of the conventional art. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.